1. Technical Field
This invention relates to the fabrication of lithographic masks and, in particular, the fabrication of scattering lithographic masks utilized in device fabrication.
2. Art Background
In the fabrication of devices, e.g., semiconductor devices or optical devices, it is generally necessary to configure on a substrate a region (e.g., a metal, semiconductor or dielectric region) in a specific spatial pattern and location. (A substrate is a mechanically stable body including, e.g., semiconductor regions and/or metal regions and/or dielectric regions formed on a supporting body such as a glass plate or on a membrane deposed across a supporting structure.) The positioning and/or patterning of these regions is generally accomplished by a lithographic process. In this process a mask is utilized to image energy in the desired pattern onto a substrate surface that has been coated with a material sensitive to the incident energy. The mask in this exposure step is, in one procedure, placed in contact with or in close spatial relation to the substrate. Alternatively, the mask pattern is projected onto the substrate.
After exposure, development of the energy sensitive material is performed to selectively remove either the exposed, or unexposed regions, of the resist material. (For a negative resist the unexposed region is removed while for a positive resist the exposed region is removed.) Generally, a solvent or energetic entities from a plasma are employed to effect this removal. The resulting, patterned energy sensitive material, i.e., resist, is employable as a processing mask for accomplishing the processing, e.g., selective doping, etching, oxidizing of or deposition onto the underlying substrate regions.
A mask designed to be used in photolithography, i.e., lithography using light in the spectral range 150 nm to 450 nm, generally includes a patterned metal or metal oxide film. Materials such as chromium, chromium oxide, tungsten, molybdenum disilicide, magnesium fluoride or nickel are typically used for photomasks. These materials are commonly formed in a layer thickness of approximately 500 Angstroms to 1000 Angstroms for photomasks on a transparent substrate such as a quartz glass substrate that is generally 0.250 inches thick. (In the context of this disclosure, the terms transparent and blocking refer to the energy that is used in inducing reaction in the resist material to be exposed. For a material region of the mask to be considered blocking, it should, in the lithographic tool, lead to an attenuation of energy reaching the substrate that is at least tenfold less than energy impacting the substrate in an equal area of the nearest region where exposure of the resist is desired. If a region is not blocking, it is considered transparent.) The metal or metal oxide film of a photomask is typically patterned by depositing a resist material sensitive to electrons or photons onto its surface, exposing this resist material with a directed electron beam or laser, developing the exposed resist to form the desired pattern and transferring the pattern using, for example, etching to the underlying metal or metal oxide layer (see, D. J. Elliott, Integrated Circuit Fabrication Technology, McGraw-Hill, N.Y., 1982, for a description of the fabrication of photomasks).
In recent years, a new form of projection electron lithography denominated SCALPEL (Scattering Angular Limited Projection Electron Lithography) has been developed. In this form of lithography, the mask has blocking and transparent regions. However, the blocking regions are built to allow a substantial level of incident electrons to traverse and emerge from the mask through scattering. (For a description of SCALPEL lithography, see L. R. Harriott, xe2x80x9cScattering with Angular Limitation Projection Electron Beam Lithography for Suboptical Lithographyxe2x80x9d, Journal of Vacuum Science and Technology, B15(6), 2130 (1997) which is hereby incorporated by reference.) The transparent regions also allow electrons to traverse the mask and emerge but induce scattering to a lesser extent. Generally a thin membrane such as a silicon nitride membrane is supported at its periphery and functions as transparent regions, while patterned metal regions such as tungsten supported on, or deposed under, the membrane (with reference to the electron source) acts as blocking regions. A filter placed at the back focal plane (or conjugate plane) of the projection lens differentiates the electrons passing through blocking regions from those passing through transparent regions of the mask. Through this differentiation, electrons either passing through the blocking regions or electrons passing through the transparent regions are allowed to reach the resist.
In the manufacture of masks, transparent defects such as pin holes or entire missing portions in blocking regions often occur. These defects, in turn, cause defects in the integrated circuit or other device produced when using the mask. Alternatively, opaque defects, i.e., unwanted blocking regions that are unintended parts of the blocking pattern, also result in defects in the final device. Additionally, for a SCALPEL mask, a pinhole in the membrane (transparent region) produces a defect that is manifested as a bright spot in the exposure image. This bright spot, depending on its location, can result in irradiation in directly adjoining regions where the image is potentially distorted.
Since the manufacture of masks is generally a time consuming and relatively expensive operation especially for scattering masks, it is often desirable to repair a defective mask by selectively forming blocking material on the unwanted transparent region or removing an unwanted blocking region. The repair procedure is, however, not acceptable unless it is less costly than merely producing another mask. The repair should also produce a blocking deposit that is sufficiently adherent to the mask substrate that subsequent processing and cleaning during mask fabrication or during subsequent use of the mask does not induce loss of the repaired material. Additionally, the resolution of the repair procedure should be at least as good as the desired resolution of the mask itself to avoid mask and, in turn, device degradation.
A variety of processes have been disclosed for effecting repair of defects. In one procedure developed for optical masks and for stencil masks employed with electron beams, repair of transparent defects is effectuated by ion beam induced reaction. In particular, a beam of gallium ions is directed at a transparent defect. An unsaturated gas such as styrene is introduced into the path of the gallium ions at the defect. The ion beam induces a reaction in the styrene that causes a carbonaceous deposit at the defect. This carbonaceous deposit has been found to be an absorber of light or electron beams and thus functions to repair transparent type defects in masks intended to prevent incident energy from traversing blocking regions. (See U.S. Pat. No. 5,273,849 which is hereby incorporated by reference.)
Repair methods for SCALPEL masks have not been reported. However, opaque defects in photolithographic masks have typically been repaired by employing ion milling. In this process, an ion beam e.g. a gallium beam, is directed at the opaque defect. Impact of the beam on the defect causes removal of the unwanted material through momentum transfer and subsequent scattering. The beam is traversed over the defect until the unwanted blocking material is removed.
SCALPEL masks, because they constitute blocking regions formed on a relatively thin transparent membrane are significantly more difficult to repair than typical photolithographic masks. The membrane is susceptible to damage that could cause mechanical failure of the membrane or a change in its thickness that leads to an unacceptable lithographic change. Thus, procedures such as ion milling present a problem associated with such damage. Additionally, use of gallium ions to mill opaque defects in photolithographic masks have resulted in the production of opaque regions in the portion of the quartz substrate bombarded by the beam after the opaque defect is removed. This undesired opacity in the quartz is removed by subsequent etching of the surface quartz to remove the substantial thickness of quartz damaged by the ion milling. Although the resulting photomask is quite acceptable, a similar remediation process for a SCALPEL mask is not acceptable because the thickness of the membrane, e.g. typically 70 to 150 nm, does not permit the required subsequent etching.
Similarly, any repair of a transparent defect including a membrane pinhole in a SCALPEL mask must have suitable density thickness and atomic number of its constituent atoms so that the mask is not lithographically compromised. Thus, any material used to repair a transparent defect must scatter to the same extent as the surrounding mask material (e.g. membrane or blocking region) rather than block the incident electrons. Thus, substantial problems are presented by the repair of SCALPEL masks relative to photolithographic masks.
It has been found that a SCALPEL mask is repairable (both transparentxe2x80x94including membrane pinholesxe2x80x94and opaque defects) by using procedures involving gallium entity beams. (A gallium entity is one that contains a gallium atom irrespective of its charge state and irrespective of how, if at all, it is bound.) Surprisingly, opaque defects are removable using a gallium entity beam without unacceptable damage to the underlying membrane and without inducing unacceptable increase in the degree of scattering induced by the repaired region. Although the beam does remove a portion of the membrane, by employing an appropriate acceleration voltage for the gallium, implantation of the gallium in the membrane occurs. It is contemplated that this implantation lithographically corrects at least in substantial part for the portion of the membrane removed during the milling process.
Equally surprisingly, the interaction of a gallium beam with styrene result in deposits that with appropriate adjustment of thickness have equivalent scattering properties to both the membrane and the blocking regions, so that effective repair is achieved without unacceptable degradation of lithographic properties. For example, a 1000 xc3x85 thick deposit formed by the interaction of a gallium beam with styrene has essentially equivalent scattering properties to a 275xc3x85 thickness of tungsten blocking region and an 80 nm thick deposit has equivalent properties to a 100 nm thick silicon nitride membrane. As a result, the repair of SCALPEL masks is possible both for transparent and opaque defects using a gallium beam either in the presence of styrene to repair transparent defects or in the absence of styrene to repair opaque defects. Thus, both opaque and transparent type defects are repairable in the same chamber without a break in vacuum. Accordingly, SCALPEL masks are efficiently repaired without unacceptable degradation of lithographic properties.